Information systems have evolved from centralized mainframe computer systems supporting a large number of users to distributed computer systems based on local area network (LAN) architectures. As the cost-to-processing-power ratios for desktop PCs and network servers have dropped precipitously, LAN systems have proved to be highly cost effective. As a result, the number of LANs and LAN-based applications has exploded.
A consequential development relating to the increased popularity of LANs has been the interconnection of remote LANs, computers, and other equipment into wide area networks (WANs) in order to make more resources available to users. However, a LAN backbone can transmit data between users at high bandwidth rates for only relatively short distances. In order to interconnect devices across large distances, different communication protocols have been developed.
Another consequential development relating to the increased popularity of LANs has been an increase in the number of applications requiring very high-speed data transmission. Applications such as video conferencing require a large amount of bandwidth to transfer data and are relatively intolerant of switching delays. The need for higher speed communication protocols that are less susceptible to switching delays has led to the development of the asynchronous transfer mode (ATM) telecommunications standard. ATM provides speeds from 50 Mbps up to 10 Gbps) using fast packet switching technology for high performance.
ATM uses small fixed-size packets, called "cells". A cell is a 53-byte packet comprising 5 bytes of header/descriptor information and a 48-byte payload of voice, data or video traffic. The header information contains routing tags and/or multi-cast group numbers that are used to configure switches in the ATM network path to deliver the cells to the final destination.
Many packet switching architectures have been developed for implementation in ATM networks. One such architecture, known as multistage interconnection network (MIN), comprises a switching fabric that routs packets that come in from one of N input ports to the appropriate one, or appropriate subset, of N output ports. The multistage interconnection network comprises groups of switching element arranged in multiple stages. Each stage uses one or more bits in the packet (or cell) header to select the output to which the input packet is routed. This type of routing is known as "self-routing".
As the data traffic in a packet switch ATM network grows, it is frequently necessary to upgrade the switches therein from N.sub.1 input/output ports to N.sub.2 input/output ports, where N.sub.2 &gt;N.sub.1. In the prior art systems, this involves a number of different options, each having distinct drawbacks. One "upgrade" method is to actually put in an over-sized fabric to begin with. Only that portion of the fabric that is needed to service the N.sub.1 inputs and N.sub.1 outputs is used. The excess switching fabric is not used until the system is upgraded to include N.sub.2 inputs and N.sub.2 outputs. This approach to upgrading results in unnecessary expense and an oversized switch for the job at hand.
Alternatively, a switch may be upgraded by shutting down the switch, removing the old fabric, and installing a new, larger fabric. The approach cannot be used in those implementations where service outages are unacceptable, such as the packet switch used by an Internet service provider.
A third approach involves the use of a redundant fabric architecture. The fabric is actually two redundant fabrics: a primary fabric and a standby fabric in parallel with one another. A redundant switch is upgraded by shutting down power in the standby fabric, removing the old standby fabric, and installing a new, larger standby fabric. Data traffic is then switched to the new standby fabric. The upgrade continues by shutting down power in the primary fabric, removing the old primary fabric, and installing a new, larger primary fabric. Data traffic may then be switched back to the new primary fabric or continue to be switched through the upgraded "former" standby fabric, which now assumes the role of the primary fabric. This approach is costly since both the primary and the standby switch fabrics are now waste.
There is therefore a need in the art for an improved packet switch architecture capable of being upgraded to a higher capacity without service disruption. There is a further need in the art for an improved packet switch architecture that initially requires only the minimum amount of switch fabric necessary to service the initial traffic requirements of the packet switch system. There is a still further need for an improved packet switch architecture that may be incrementally upgraded to handle higher data traffic capacities with a minimum amount of waste due to fabric replacement.